Using v-sis-transformed fibroblast cell lines, we were able to demonstrate that both alpha and beta platelet-derived growth factor (PDGF) receptors are activated in an intracellular compartment. Yet a surface localization is required for coupling the activated receptors to a mitogenic response. These findings served as a useful model system to investigate human tumor cell lines and understand the role PDGF isoforms may have in the neoplastic process. Additionally. our studies indicate that the drug suramin, which can completely abrogate the transformed phenotype in sis/3T3 transformed cell lines. is able to significantly alter the proliferation of many human tumor cell lines that express either alpha or beta PDGF receptors and the A or B chain of PDGF.